1. Field of the Invention
This invention generally relates to wireless networks and to IP Multimedia Subsystem (IMS) networks.
2. Description of Related Art
Current wireless networks support circuit-switched (CS) and packet-switched (PS) connections. In some wireless networks, both types of connections may exist contemporaneously and be available to mobile handsets or user endpoints (UEs). In other wireless networks, a mobile handset may have access to either a CS connection or a PS connection but not both at the same time. Moreover, even if a mobile handset has a PS connection, it may lose packet connectivity due to a policy implemented by a network operator (e.g., in a situation where the operator has a limited number of IP addresses that need to be shared amongst a large number of handsets). In this case, because the mobile handset is no longer assigned an IP address, it is not reachable by addressing schemes based on IP addresses, i.e., it is not IP-accessible. In such situations, IP-based user-to-user signaling schemes such as SIP (Session Initiation Protocol) may become inoperable because one or more user handsets are no longer IP accessible.
CS and PS networks will now be described in greater detail. In a CS network such as PLMN, users' network mobile handsets are connected to Base Transceiver Stations (BTS) through a radio access network. The BTS in turn are connected to a plurality of Base Station Servers (BSC) that in turn are connected to a network of Mobile Switching Centers (MSC). The MSC provide wireless services to the users' handsets, and are also inter-connected with the Public Switched Telephone network (PSTN). This arrangement makes it possible for voice traffic to be carried between mobile handsets and landline telephone sets. The MSC in a wireless network effectively behaves as a switch that supports the mobility and roaming functions of a user's handset.
When a user's handset requests a telephone call or a service, such as voice mail, a prepaid call, or a toll-free call, it generates a “call event” at the MSC. Each call event can potentially “trigger” one or more Trigger Detection Points (TDP) in the MSC. When a call event triggers a particular TDP, the MSC sends a pre-specified message to a Service Control Function (SCF). The message includes, for example, the phone numbers of the calling and called parties, and the nature of the service request. The SCF then “fields” the message, i.e., service logic within the SCF responds appropriately to the message. In WIN/CAMEL implementations, the MSC and SCF communicate using standards-based protocols such as Transaction Capabilities Application Part (TCAP) from the family of protocols commonly referred to as Signaling System 7 (SS7).
For example, consider a “call origination” call event that happens when a user makes a new call request at the MSC. This call event triggers a corresponding TDP, causing the MSC to send a message with event-related information to the SCF, e.g., the calling and called numbers. The SCF then processes the message, e.g., by querying an internal or external database to verify that the calling party is authorized to initiate telephone calls. The SCF then responds back to the MSC with a message that indicates whether the call is “allowed” or “denied.”
In a PS network, services are generally supported by IP Multimedia Subsystem (IMS). The IMS architecture manages the network with several control functions, i.e., functional entities. The Breakout Gateway Control Function (BGCF) is an inter-working function that handles legacy circuit-switched traffic. A new function called the Media Gateway Control Function (MGCF) controls the Media Gateway (MGW). The Media Resource Function Processor (MRFP), which is controlled by the Media Resource Control Function (MRFC), performs media processing functions. An IMS session is controlled by a logical function called the Call State Control Function (CSCF). It is logically partitioned into three functional entities, the Proxy, Interrogating and Serving CSCFs. The Proxy Call State Control Function (P-CSCF) is the first contact point for a user's handset. The Interrogating CSCF (I-CSCF) is mainly the contact point within an operator's network for all IMS connections destined to a subscriber of that network operator, or a roaming subscriber currently located within that network operator's service area. The Serving CSCF (S-CSCF) actually handles the session states in the network. “Third party” application servers (AS) provide services to the mobile handset, such as voice mail, via the S-CSCF. The IMS controls packet services among the different functional entities with signaling protocols such as Session Initiation Protocol (SIP), which is an IP-based signaling protocol designed for multimedia communications.
When a mobile handset first powers on, logic residing in the handset initiates a “registration” procedure with the IMS core, first by requesting the radio access network to assign it an IP address. After it receives an IP address, the mobile handset attempts to register as an IP-enabled endpoint with the IMS core, by sending a “register” request to the P-CSCF. Assuming that the handset is registering from a visiting domain, the P-CSCF then uses a Domain Name Server (DNS) to search for the handset's home domain S-CSCF. Once the P-CSCF locates the S-CSCF for the mobile handset, it passes the “register” request to that S-CSCF. The S-CSCF contacts the Home Subscriber Subsystem (HSS), which looks up the mobile handset's profile. This profile contains assorted information about the user, and what services the handset is authorized to use. A logical function in the S-CSCF called the “registrar” then authenticates the mobile handset, e.g., verifies that the handset is legitimate.
The S-CSCF also loads Service Point Triggers (SPT) from the handset's profile. The SPT define the appropriate action for the S-CSCF to take when the handset or an AS requests a transaction. For example, if the handset requests voice mail service, the SPT triggers the S-CSCF to provide the addresses of the voice mail AS for the handset. So long as the handset is powered on, the SPT for that handset are loaded into the S-CSCF, so a service request fires the appropriate trigger in the S-CSCF. The SPT are analogous to the above-described TDP in the CS network. The SPT and TDP both trigger an appropriate response from a controlling server, e.g., the MSC or S-CSCF. However, the TDP are more generally applicable to call requests and call related events such as dialed number, etc., and are not particular to the user's profile. The SPT are specific to the mobile handset, and are stored in the user's profile in the HSS and loaded into the S-CSCF when the handset registers.
If an entity wishes to engage in a transaction with the mobile handset, e.g., to send a message to the handset, the entity utilizes an AS to send a request for the transaction to the S-CSCF. This triggers an SPT in the S-CSCF, which recognizes the request as pertaining to a registered handset and sends the appropriate information to the handset. Other ASs may not know which S-CSCF to contact in order to engage in a transaction with a particular handset. In this case, the AS interrogate a Subscriber Location Function (SLF), which provides information about a handset's S-CSCF to the AS, which then contacts that S-CSCF as described above. If the handset wishes to request a service, it sends the request to the S-CSCF, e.g., using a SIP invite. This triggers an SPT in the S-CSCF, which then directs the service request to a particular Application Server (AS), which then provides the service to the handset. For example, if the user wants to initiate an IMS call, it sends a SIP invite message to the S-CSCF, which may then contact the AS responsible for IMS calls, called the Back-to-Back User Agent (B2BUA), which initiates the IMS call flow.